A three-justice panel of the 9th U.S. Circuit Court of Appeals on Tuesday heard oral arguments about the constitutionality of California’s “Low Carbon Fuel Standard,” a piece of the state’s landmark global warming law, AB 32.

The justices’ questions focused on the law’s reliance on a “carbon intensity score,” which measures pollution from a fuel’s entire life cycle, not just when it is burned in a vehicle.

Out-of-state refiners and ethanol companies say this discriminates against their products because transportation to California raises the score.

The state says the law does not discriminate and will spur clean-fuels innovation.